This invention relates to a color image processing method and an apparatus utilizing the same and, more particularly, to a color image processing method and an apparatus utilizing the same for obtaining identical color reproduction characteristics between apparatuses, e.g., a color image input device, a display device, and an output device, having different color reproduction characteristics from each other.
Conventionally, a CRT color monitor reproduces an image by using additive color mixing in which luminance signals, for each RGB phosphor, are changed, whereas a color printer reproduces an image by using subtractive color mixing in which ink density of each YMCK color is changed. Therefore, when an image displayed on such a CRT color monitor is reproduced by an ink-jet type color printer, for instance, the color displayed on the monitor and the color outputted by the color printer are quite different from each other in most cases because of the different color reproduction characteristics of these two devices.
Further, since a color reproducible range (color gamut) of a printer is much smaller than that of a color monitor because the principles of the color reproduction methods of these two devices differ, it is impossible to reproduce all the colors displayed on the color monitor by the printer.
As a result, although an image is displayed on the color monitor with different tones, the printer may output an image that is deteriorated and has fewer tones, which has been a conventional problem.
For the sake of solving the problem, there are proposed a plurality of methods of obtaining identical color reproduction characteristics by analyzing the color reproduction characteristics of the monitor and the printer in advance and compensating for color differences between the two devices (color matching process).
As an example of those methods, there is one shown in FIG. 12. In FIG. 12, reference numeral 71 denotes a first conversion function for converting a first color signal representing a color on a monitor into a standard color signal, and reference numeral 72 denotes data showing color reproduction characteristics of the monitor analyzed in advance. Coefficients of the first conversion function are determined based on the data. Note, a standard color space signal 73 is expressed in the XYZ color system defined by the CIE (Comission Internationale de l""Eclairage) in 1931. Further, reference numeral 74 denotes a second conversion function for converting the standard color space signal 73 expressed by the XYZ color system into a second color signal for printer output, and reference numeral 75 denotes data showing color reproduction characteristics of the printer. The second conversion function is determined in accordance with the data.
More specifically, the idea illustrated in FIG. 12 is to convert an image on the monitor into an image outputted by the printer having an identical color reproduction characteristics to those of the monitor by using the two conversion functions via the standard color space signal which is expressed in the XYZ color system.
In the aforesaid method, however, a color signal is converted twice by operating the two conversion functions, thus it requires considerable processing time. It is possible to make another function by synthesizing the two functions in advance, then perform conversion of the color signal. However, when characteristics of a device to be used are changed or a conversion result is to be outputted to another device having different output characteristics, the synthesized function must be generated each time, which makes an operational sequence of the image processing complicated.
Furthermore, when the XYZ color system is employed as the standard color space, there is a problem in that quantization error during signal conversion becomes large, which causes deterioration of the image.
In addition, since the second conversion function includes a process to adjust differences between the color gamuts of devices, it is difficult to change only part of the adjusting process, or select the adjusting process out of plural kinds of processes.
Accordingly, it is an object of the present invention to provide a color image processing apparatus capable of performing color conversion flexibly by easily adjusting differences of color reproduction characteristics between different devices.
According to the present invention, the foregoing object is attained by providing a color image processing apparatus comprising: memory means for storing a first conversion function for converting a first color image signal into a signal in a predetermined color space and a second conversion function for converting the signal in the predetermined color space into a second color image signal, and a third conversion function used for performing a color signal conversion in consideration with color gamuts of units, which output the first and the second color image signals, in the predetermined color space; first synthesize means for obtaining a fourth conversion function by synthesizing the third conversion function and at least the first or the second conversion functions; and first conversion means for converting the first color image signal into the second color image signal by operating the fourth conversion function obtained by the first synthesize means.
It is another object of the present invention to provide a color image processing method of performing color conversion flexibly by easily adjusting differences of color reproduction characteristics between different devices.
According to the present invention, the foregoing object is attained by providing a color image processing method, based on a first conversion function used for converting a first color image signal into a signal in a predetermined color space, a second conversion function used for converting the signal in the predetermined color space into a second color image signal, and a third conversion function used for performing color signal conversion in consideration with color gamuts, in the predetermined color space, of units which output the first and the second color image signals, where all of the functions are stored in a storage medium, comprising: synthesize step of obtaining a fourth conversion function by synthesizing the third conversion function and at least the first or the second conversion functions; and conversion step of converting the first color image signal into the second color image signal by operating the fourth conversion function obtained in the synthesize step.
In accordance with the present invention as described above, in order to adjust the differences between color gamut of the first color image signal and color gamut of the second color image signal, by using the first conversion function for converting the first color image signal into the signal in a predetermined color space and the second conversion function for converting the signal in the predetermined color space into the second color image signal, and the third conversion function used for performing the color signal conversion in consideration with color gamuts of units which output the first and the second color image signals in the predetermined color space, a fourth conversion function is obtained by synthesizing the third conversion function and at least the first or the second conversion functions, and the first color image signal is converted into the second color image signal by operating the fourth conversion function.
The invention is particularly advantageous since the adjustment of the differences between the color gamut including the first color image signal and the color gamut including the second color image signal is performed by using the third conversion function, the adjustment can be easily and more flexibly performed by only changing, adding, or selecting the third conversion function. Further, selection designating processes regarding color characteristics of input image signal, a method of color matching process, and color characteristics of an output image signal are performed independently, thus it is easy to select, add, and change each process.
As a result, it becomes possible to arrange a system for effectively adjusting color reproduction characteristics between different devices.